Hello Jen!
Thank you so much for your review; I'm so sorry to hear you had a poor result when trying the product.
If you used the Very Dark Brown over already dark hair, the likely result would be that it would darken further.
Colour issues such as blue, green or purple tones tend to come from the Indigo in the dye, either because the Henna in the dye hasn't taken so well, or because it has built up over time - but this is absolutely avoidable, and something you can colour correct, either by using a different dye or using the dye in a different way.
I appreciate it can be really tricky to find the right colour & get the shade you're hoping for; which is why doing a strand test was the best thing to do!
I'm happy to help you with finding the right product for you if you're able to get in touch via our contact us page.
With Kindest Regards
Rebecca